Refactor demand share module

[tsmbland]

This is a refactor the the demand_share module which I think makes the code more readable and leaves less room for bugs. It also removes legacy code to do with the "forecast" feature, as we now only have the current year and investment year.

Main changes:

Most functions in this module were previously taking "year" and "forecast" as arguments, along with a market object that should cover this time period. We're getting rid of the forecast feature, so the forecast year is essentially always the investment year. We can then simplify this the code by extracting the year range from the market object: the first year is the current year, and the second year is the investment year.

As a result, there is less need for interpolation in some places as the market object always covers the appropriate years. This is good, as I think we should rely on interpolation sparingly (as this is essentially making up data), and be super explicit when we do need to interpolate.

There are also checks in place to ensure that object dimensions are as expected:

• technology parameters should be for a single year (no "year" dimension)
• market objects should (usually) cover two years: current year and investment year
• same (usually) for capacity data

I've moved decommissioning_demand to the demand_share module as this seems like a more sensible place for it to belong

I've updated the tests, but they're still a mess to be honest and could do with a complete rewrite

In working through this I spotted a bug which I've market with a TODO - I'll fix this in a separate PR as I wanted this PR to be a pure refactor (not changing any results)